The present invention relates to a signal generating apparatus, and more particularly to a phase-locked loop based transmitter with a loop bandwidth compensation scheme, wherein the loop bandwidth compensation scheme calibrates a phase/frequency charge pump device according to a synthesized signal of a controllable oscillator of a signal generating apparatus, and a method thereof.
In general, phase locked loop (PLL) circuits are utilized for synthesizing a desired output signal having a frequency that is a multiple of a reference frequency from a source signal. The loop bandwidth of the PLL circuit is maintained as stably as possible within a predetermined range of output frequency of the PLL circuit. Please refer to FIG. 1. FIG. 1 is a related art Sigma-delta fractional N phase locked loop (PLL) transmitter 10. The Sigma-delta fractional N PLL transmitter 10 comprises a phase locked loop circuit 11, a Sigma-delta modulator 12, a channel selector 13, a Gaussian filter 14, and a compensation filter 15. The phase locked loop circuit 11 comprises a phase/frequency detector 11a, a charge pump circuit 11b, a loop filter 11c, a voltage-controlled oscillator 11d, and a divider 11e. Baseband data Sb is inputted to the Gaussian filter 14 to extract the required band of the Sigma-delta fractional N PLL transmitter 10. For example, if the transmitted signal of the Sigma-delta fractional N PLL transmitter 10 is a Gaussian Minimum Shift Keying (GMSK) signal SGMSK, then the Gaussian filter 14 is a GMSK filter. Furthermore, as the frequency response of the phase locked loop circuit 11 acts like a low pass filter, the compensation filter 15 is utilized to compensate the baseband data Sb before it is modulated by the Sigma-delta modulator 12. The phase locked loop circuit 11 utilizes a reference frequency Sr to synthesize the required frequency to transmit the baseband data Sb. In a multi-band system, the phase locked loop circuit 11 needs to generate different frequency bands, and the channel selector 13 is utilized to select the required frequency band. Through the variation of the dividing number utilized by the divider 11e, the phase locked loop circuit 11 can generate different frequency bands accordingly. However, the loop bandwidth is sensitive to variations of parameters in the transfer function of the phase locked loop circuit 11. Therefore, an effective scheme to calibrate the loop bandwidth of the Sigma-delta fractional N phase locked loop (PLL) transmitter 10 is necessary. For the full description of related techniques, U.S. Pat. No. 6,724,265 B2 can be referred to.